This invention relates to shanks for knee-joint endoprostheses of the kind which, when the prosthesis is being implanted, is inserted in a space prepared in the spongiose and marrow of the bone which is to receive the shank.
The shanks of known knee-joint endoprostheses are formed to be square or approximately square in cross-section with or without longitudinal flutes or splines, and they end in a more or less pronounced point. Their diameter is relatively small, being 8 to 10 mm for example, and is constant throughout their length and their length is 15 to 20 times the thickness of the shank. When prostheses having such shanks are implanted, the shanks reach a very long way into the medullar cavities in the thigh bone and shin bone of a human being. To ensure that the shanks are adequately anchored in the spongiose and in the medullar cavity, a cementing substance is used when the shanks are inserted.
The fact that the shanks of known knee-joint endoprostheses are relatively long and come to a point is a major disadvantage because they often damage the bone surrounding the medullar cavity when they are implanted, even though they are shaped into a curve to follow the slight curvature of the bone. Because of their slight curvature they cannot be inserted centrally into the bone, or can only be so inserted with difficulty, thus making it difficult to accurately position the endoprosthesis being implanted. Also, shaped shanks have the disadvantage that if prostheses are removed, the shanks can only be freed by damaging the end parts of the thigh and shin bones next to the knee joint; freeing them thus involves a loss of bone material. Also, shanks of square or approximately square cross-section do not provide adequate security against the implanted prosthesis turning, particularly during the phase of the operation when it is being cemented into place.
It is therefore an object of the invention to provide shanks, which can be implanted accurately in thigh and shin bones, for knee-joint endoprostheses in such a way as to be secure against turning, avoid damage to the bone, and are relatively simple to free again from the bone material.
This and other objects are achieved with shanks of the above mentioned kind, by matching the contour of the shank, which preferably tapers in the direction of its free end, for at least part of the length of the shank to the cross-section of the medullar cavity in a normal human bone which is to receive the shank.
Advantageously the shank is formed as a part separate from the remainder of the knee-joint endoprosthesis and, in its end portion nearer the knee-joint, is provided with a tapering axial bore to accept a conical spigot on the joint-part of the prosthesis. From the bottom of the tapering bore there extends an internally threaded axial hole in which an extracting tool can be engaged. The end of the shank nearer the joint has a flange-like collar with support faces.
A shank formed as above is relatively short and it is ensured that it can be centrally and non-rotatably inserted in the bone because its external contour in cross-section is matched to the cross-sectional shape of the medullar cavity in the thigh or shin bone. In addition, it allows damage to the bone to be avoided and can easily be freed from the bone since, in addition to being relatively short, it tapers, preferably uniformly, to its free end. The diameter of the shank is made sufficiently small at all points to provide a clearance between it and the remaining bone material, so that allowance can be made for individual differences between bones and so that adequate space is available for the cement used to anchor the shank.
The length of the steadily tapering section of the shank, which, in accordance with the invention, is of an anatomically matched shape, is advantageously 8 cm. This makes it possible to use only one type of shank since there is no need to make provision for individual differences in the thigh and shin bones. The principal used to anchor the shank is that of taper wedging. In principle the shank has a smooth surface, so that it can very easily be freed again by screwing a suitable tool into the aforementioned internally threaded hole in the shank and removing the shank by for example tapping the tool backwards. The basic cross-sectional shape of the shank according to the invention remains an important factor even when the shank is provided with support faces or other anchoring means to allow it to be anchored in the shaft of the bone without cement.